This invention relates to end-to-end connection of optical fibers, and more particularly, to a variable attenuator for use in such connections.
Fiber optics has become, in most cases, the preferred mode of signal transmission, especially where the signals are in the higher frequencies. Optical fibers produce greatly increased bandwidth over conventional electrical conductors, and are relatively immune to ambient conditions that can disrupt electrical signal transmission. The numerous advantages of optical fibers come with a price, however. Whereas with electrical conductors, splicing may be had by simply butt welding, soldering, or otherwise joining the ends of the conductors being spliced, such is not presently possible with optical fibers, which have a diameter of, for example 125 microns and a core diameter of from 6 to 30 microns, and, in a satisfactory splice, must permit light transmission with a minimum of insertion loss while providing a stable junction. As a consequence, there has been a constant and ongoing effort to achieve, in a connector splice arrangement, an alignment of the butting ends of the fibers to minimize insertion loss. In addition to alignment, the width of the gap between the fiber ends and the surface condition of the ends are factors which must be considered in reducing or minimizing loss. The prior art arrangements are, for the most part, dependent upon or directed toward the centering of the fiber cores, and in most instances, the fibers are contained in ferrules which have centered fiber containing bores therein. On the other hand, in U.S. Pat. No. 4,544,234 of DeVeau et al., the fibers are contained in a slotted tube and centered prior to their being cemented in place within the slot. Such an arrangement does not require centering of the bores of the ferrules, but it does not permit relative movement between the fibers for optimum transmission after the fibers are cemented in place.
In U.S. Pat. No. 4,691,986 of Aberson et al., the disclosure of which is incorporated herein by reference, there is shown an arrangement wherein alignment of the fiber containing bores of plugs or ferrules is achieved, thereby obviating at least to a large extent alignment of the fiber cores. The invention of that patent involves the use of xe2x80x9ccontiguousxe2x80x9d plugs derived from contiguous segments of tubular stock, with the contiguous ends being the mating ends, with the rotational relationship between the plugs being that that existed prior to cutting or sectioning of the tubular stock. With such an arrangement, the fiber containing bores are automatically aligned. It doesn""t matter if the bores are slightly eccentric relative to the outer diameter of the plugs, they will still be aligned, thereby enabling alignment of the fiber cores. The invention of that patent is usable in almost any of the large variety of prior art connectors wherein the fibers are contained within ferrules or plugs.
The foregoing is directed to the general problems inherent in most fiber connectors of achieving proper fiber alignment in connectors or splices. These problems also exist in various optical fiber components, such as, for example, variable attentuators. In U.S. Pat. No. 4,986,627 of Boscher et al., there is shown a variable attenuator arrangement that has abutting ferrules having substantially identical outer diameters, but with fiber containing bores which are eccentric with respect to the other diameters. The bores, each of which has a diameter substantially the same as the outer diameter of the fiber contained therein, are rotatable relative to each other to vary the alignment of the fibers relative to each other and thereby to vary the attenuation by increasing or decreasing the offset between the fiber cores. Such an arrangement requires, for optimum performance, that, at at least one rotational position, the ferrule bores are substantially aligned, thereby aligning the fiber cores at least to the extent possible with whatever eccentricity may exist for the fiber cores themselves. The Boscher et al. arrangement produces an attenuation coefficient which varies between 3 and 60 dB for fibers which are off center by 7 to 10 microns (xcexcm) for single mode fibers. For multimode fibers, the offset from center is from 30 xcexcm to 50 xcexcm.
In addition to arrangements such as shown in Boscher et al, other prior art variable attenuators make use of air gaps and/or angled ferrule (and fiber) ends. Ideally, a variable attenuator should provide the needed or desired attenuation, should have low return loss, and should have stable performance in high power systems and should comprise a single unit that can be inserted within a transmission system between two fiber ends.
The present invention, hereinafter shown as embodied in a variable attenuator, overcomes or obviates many of the shortcomings of the prior art, as enumerated hereinbefore, while providing reliable desired variations in the attenuation of signals passing therethrough. It should be recognized that in many instances some level of attenuation may be desirable in order to achieve a power balance among several related transmission lines rather than simply a maximum signal power throughput.
The variable attenuator embodying the principles of the invention comprises a fiber connector having a ferrule or glass capillary mounted in a stationary holder and a butting ferrule mounted in a rotatable holder. Glass or similar material ferrules have distinct advantages over ceramic or other material ferrules. A glass ferrule spreads the energy over a large area, thereby preventing energy absorption at the ferrule end face. For high power applications this directing energy away from the end face prevents long term reductions in performance due to heat concentration. The centerlines of the two, substantially identical, ferrules are aligned, and each ferrule has a fiber containing bore extending therethrough, the bores being offset from the ferrule centerline by equal amounts, approximately 35 xcexcm to 40 xcexcm. The ferrules are contained in barrel members and their distal ends are spaced from each other by a gap of approximately 5xcexc to 15xcexc which is filled with an index matching material. The ferrules are contained in a sleeve of suitable material such as ceramic or metal whose ends butt against the two ferrule containing barrels and which aligns the centerlines of the two ferrules. The sleeve length is the determinant for the width of the gap when the ferrules are completely inserted therein.
Prior to final assembly, the ferrule end faces are polished, either normal to or at an angle to the centerline thereof and the barrels function as stops for the polishing mechanism, thereby insuring that the front face of each ferrule is a desired distance from the front face of the barrel. Thus, the barrel and sleeve together provide a consistent gap which may be normal to or at an angle to the centerline between the distal ends of the ferrule in the production of the attenuators, insuring consistent results among the several attenuators thus produced.
In assembly of the barrels, ferrules, and sleeve, virtual identity of the ferrules is assured by mounting a glass rod of proper diameter and having an offset bore within and extending between the barrels. The rod is then cleaved at the middle thereof, creating two, substantially identical ferrules. The barrels are keyed in a housing against accidental rotation, so that when each barrel/ferrule assembly, when removed from the housing and ferrule distal ends are polished, is replaced in the housing, and the offset bores are in approximately perfect alignment.
One of the ferrules/barrel assemblies is made rotatable with respect to the other by suitable rotating means. Initially, with both barrels keyed in the housing, the offset bores are aligned, as are the ends of the fibers contained therein, and there is substantially no attenuation (0 dB) present in the transmission line into which the attenuator is inserted. Rotation of the rotatable barrel moves the fiber ends out of registry, thereby attenuating the signal within the transmission line. In practice, it has been found that approximately 30xc2x0 of rotation away from alignment produces approximately 60 dB of attenuation. Thus, the attenuator is capable of producing a range of attenuation from 0 to xe2x88x9260 dB.
These and other features of the present invention will be readily apparent from the following detailed description, read in conjunction with the accompanying drawings.